A styrenic polymer essentially having a syndiotactic structure and a composition containing tine polymer, which have been developed recently, are widely known as engineering plastics with good heat resistance and good chemical resistance. The polymer and the composition containing it will be hereinunder referred to as SPS. SPS has various applications in various molding methods. For example, it is molded into films, sheets and the like through extrusion molding.
Extrusion molding of SPS pellets having a low degree of crystallization is problematic in that the SPS pellets as heated at the glass transition temperature of SPS or higher are softened and aggregated in the feed zone near the inlet of the extrusion-molding machine and that the softened pellets adhere to the screws in the extrusion-molding machine and even to the inner wall of the barrel of the machine. As a result, it is often difficult to constantly feed the SPS pellets to the extrusion-molding machine, or as the case may be, the screws are often stopped.
One measure to solve these problems is disclosed in Japanese Patent Application Laid-Open No. 02-194044, in which they say that the SPS pellets to be fed to an extruder shall have a specifically-controlled melt viscosity and a specifically-controlled degree of crystallinity. A method for producing such SPS pellets is disclosed in JP-A 02-194044, which comprises melt-extruding SPS, and then cooling the resulting SPS strands in air or cooling them in water followed by re-heating the resulting SPS pellets.
However, the process comprising the step of cooling the SPS strands in water followed by the step of re-heating the resulting SPS pellets, which is to increase the degree or crystallinity of SPS, is complicated. On the other hand, the process comprising the step of melt-extruding SPS followed by the step of cooling the resulting SPS strands in air is simple, but is problematic in that the SPS strands are often cut in some unsuitable cooling conditions, and that the degree of crystallinity of the SPS pellets obtained often differs or could not be satisfactorily increased, depending on the cooling conditions employed. Thus, in this process, it is difficult to stably produce SPS pellets having a high degree of crystallinity.
Accordingly, it is greatly desired to develop a method for stably and efficiently producing SPS pellets with a high degree of crystallinity that are suitable for molding, especially for extrusion molding.